1. Field of the Invention
The present invention relates to a method of measuring film thickness of the film layer and more particularly to a method of measuring thickness of various kind of film on a certain material by measuring spectral reflectivity.
2. Description of the Prior Art
When light beam is emitted to material of which film thickness is to be measured, reflection interference takes place between light beam reflected at the surface of the film layer and light beam reflected at the interface between the film layer and the base plate and thus reflected light beam reaches the extreme value of intensity when it has a certain specific wavelength. At this moment the operating condition is generally represented by the following formula. EQU 2nd=m.lambda. (1)
where the above operating condition represents the case when intensity of reflected light beam assumes the maximum value. EQU 2nd=(m+178).lambda. (2)
where the above operating condition represents the case when intensity of reflected light beam assumes the minimum value.
In the above-noted formulas reference letter n designates refractive index, reference letter d designates the film thickness, reference letter m designates the order of interference and reference letter .lambda. designates the wavelength of the light beam.
Now, it is assumed that in the formula (1) the orders corresponding to extreme values (either maximum or minimum one) are represented by m.sub.1 and m.sub.2 and wavelength associated with the orders are represented by .lambda..sub.1 and .lambda..sub.2 (where .lambda..sub.2 &lt;.lambda..sub.1). Thus, the formula (1) is modified into the following two formulas. EQU 2nd=m.sub.1 .lambda..sub.1 ( 3) EQU 2nd=m.sub.2 .lambda..sub.2 ( 4)
Incidentally, the film layer concerned with the invention is a film layer on the base plate such as silicon wafer or the like on which light beam is scattered and absorbed very few. When the orders m.sub.1 and m.sub.2 and wavelength .lambda..sub.1 and .lambda..sub.2 are determined, it results that film thickness d can be determined. On the other hand, the following formula is obtained from the formulas (3) and (4). ##EQU1##
Since the following relation is existent between the adjacent maximum values (peaks) of intensity of reflected light or between the adjacent minimum values (dips) in the formula (5), namely EQU m.sub.2 -m.sub.1 =1
the following formula (6) is obtained. ##EQU2##
When film thickness is measured by utilizing the formula (6), there is no necessity for obtaining absolute value of the orders m.sub.1 and m.sub.2. In this case, however, measuring of film thickness can be easily achieved by scanning wavelength of monochromatic light and thereby measuring intensity spectrum of reflected light.
However, it is found that the conventional method of measuring film thickness by using the formula (6) has a problem that film thickness d is measured with lower accuracy because of the fact that the formula (6) contains two values of .lambda..sub.1 and .lambda..sub.2 with measuring error unavoidably included therein.